1. Technical Field of the Invention
This invention is related generally to wireless communication systems, and more particularly to receiver architectures in wireless communication systems.
2. Description of the Related Art
Communication systems support wireless and wire lined communications between wireless and/or wire-lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks to radio frequency identification (RFID) systems. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), wideband CMDA (WCDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution service (MMDS), RFID protocols and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, RFID device or other handheld device, communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switched telephone network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.) that performs analog signal processing tasks as a part of converting data to a radio frequency (RF) signal for transmission and a received RF signal to data. Most communication systems employ different RF frequency bands for transmit and receive. However, some communication systems utilize the same frequency band for transmit and receive.
In cellular communication large blockers are present at the receivers along with the desired signals. The blockers are dominated by self transmitters in full division duplex (FDD) systems, such as WCDMA, since the transmitter and receiver are ‘ON’ simultaneously. However in time division duplex (TDD) systems, such as GSM, the blockers are fRom other users' transmitters. The blockers could degrade the sensitivity of the receiver, most likely in three ways: first they could saturate the RX, secondly they could inter-modulate with strong jammers to generate in-band cross-modulation distortion (XMD), and thirdly they might generate low-frequency 2nd-order inter-modulation distortion (IMD2) at baseband along with the desired received signal. Therefore blocking performance imposes stringent requirements on the integrated receiver designs.
Conventional WCDMA systems deploy off-chip duplex filters before receiver input to reject the out-of-band blockers by 45-55 dB on average, and also to scale down the TX power amplifier (PA) noise floor to at least 10 dB below the thermal noise (kTB in 3.84 MHz bandwidth). To further lower the distortions generated by blockers off-chip RF surface acoustic wave (SAW) filters with typical 20-25 dB blocker rejection are deployed in receiver path between LNA and mixer.
SAW filters, however, introduce several drawbacks: first they have 2-3 dB insertion loss at desired received band. Secondly, the LNA output needs to be matched to the input impedance of SAW filter, 50Ω. To compensate for lower load resistance, LNA consumes more bias current to retain the high gain. Thirdly the output of SAW filter needs to be matched to the input impedance of the proceeded stage, which is typically another LNA. The second LNA compensates the insertion loss of SAW filter in RX band, and also lowers the mixer noise. Finally the SAW filters are off-chip components, which degrade the integration level of transceiver and increase its cost.
Currently, CMOS process fails to provide feasible on-chip inductors with high quality factors (i.e. 90-100) to achieve minimum blocker rejection of 20 dB, at tens of mega hertz away from desired signal band (for example 190 MHz frequency spacing in WCDMA). However, a simple RC lowpass filter easily provides 20 dB of rejection in a decade away from the LPF corner frequency. One known solution is to filter blockers at DC or low IF, but, with the blocker filtering after the down conversion mixers, the receiver and/or the down conversion mixers could saturate.
Therefore, a need exists for an architecture capable of canceling blocking signals.